docs: document GitLab auth for private Go modules

[k4l397]

Changes

• Updates documentation to describe required setup for resolving modules in private gitlab repositories

Context

Private GitLab projects need a hostType: "go" host rule with a username and password so Renovate can authenticate the go-get request it uses to resolve a module's source repository. Without it GitLab returns a truncated go-import path for the unauthenticated request and the version lookup 404s.

Came up in discussion #37624 after the resolution change in #36963.

Please select one of the following:

• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):
  Used to verify what I wrote was accurate since it's been a while since I looked at this area - e.g. quick tests of the ?go-get behaviour. I did carefully review the output and have cut down a lot of what was written to what I considered useful to a user.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests, but ran on a real repository, or
• Both unit tests + ran on a real repository

No code changes - verified documentation as part of investigation in the linked discussion.